Electrical measuring instruments, such as multimeters, employ measurement leads to couple the instrument to an electrical circuit or other item being tested. Measurement leads include connector pins on one side that are designed to be inserted into receptacles that form part of the electrical measuring instrument Usually, an electrical measuring instrument has multiple receptacles, with one set of receptacles used during a voltage measurement, and another set of receptacles used during a current measurement. During current measurement, the voltage across the current measurement set of receptacles is low. Because the impedance of the measurement circuitry connected to the current measurement receptacles is very low, applying a large voltage to the current measurement receptacles may result in a very high current flowing through the current measurement circuitry, and the electrical measuring instrument may be damaged or destroyed. Therefore, it is highly desirable for an instrument to warn a user that a measurement lead is installed in the current measurement input receptacle so the user does not mistakenly attempt to measure voltage when the measurement leads are inserted into the electrical current measurement receptacles This is usually accomplished by using a split jack receptacle and a measurement lead detect circuit. Additionally, an electrical meter equipped with measurement lead detection capability can warn a user of an improperly seated measurement lead or a blown overcurrent fuse.
A split jack receptacle includes a pair of spaced-apart electrical contacts. The insertion of a connector pin located at one end of a measurement lead into a split jack receptacle bridges the space between the contacts thereby creating an electrical connection between the contacts.
Measurement lead detect circuits commonly consist of a shunt circuit and a detection circuit separated by a split jack receptacle used for electrical current measurements. A common method of realizing a measurement lead detect circuit is to couple one contact of a split jack receptacle to ground through a low-value shunt resistor in series with an overcurrent fuse, and couple the other contact of the split jack receptacle to a supply voltage through two series connected high-value pull-up resistors. The voltage at the common node of the two high-value pull-up resistors is provided to detection circuitry located within the measuring instrument by connecting the common node to a high-input impedance device, such as an analog-to-digital converter or a comparator. The detection circuitry located within the measuring instrument is designed to indicate the presence of a measurement lead when the voltage at the common node of the two high-value pull-up resistors is less than the supply voltage, and indicate the absence of a measurement lead when the voltage at the common node of the two high-value pull-up resistors is equal to the supply voltage.
When a measurement lead pin is inserted into the split jack, the supply voltage is coupled to ground through the two high-value pull-up resistors in series with the low-value shunt resistor and overcurrent fuse, resulting in electrical current flow. Current flow through the two high-value pull-up resistors results in a voltage drop across the resistors. This voltage drop divides the supply voltage between the two high-value pull-up resistors. The resulting voltage at the common node of the two high-value pull-up resistors is therefore less than the supply voltage, which indicates the presence of a measurement lead in the split jack. Conversely, if no measurement lead is present, the supply voltage and the two high-value pull-up resistors are not coupled to ground, and there is no electrical current flow As a result, there is no voltage drop across the two high-value pull-up resistors. The voltage at the common node of the two high-value pull-up resistors is therefore equal to the supply voltage, indicating the absence of a measurement lead.
When a measurement lead is properly seated in the split jack, the electrical measuring instrument may be used to measure electrical current flow through an item being tested. The other end of the measurement lead is connected to the item being tested, and the electrical current is coupled through the lead to the electrical measuring instrument, through the overcurrent fuse and shunt resistor, and returns through a common measurement lead. Current injected into the electrical measuring instrument via the measurement lead by the item being tested is divided between the shunt and the voltage divider. Because the shunt impedance is several orders of magnitude less than the voltage divider impedance, nearly all injected current flows through the shunt. The current flowing through the shunt creates a proportional voltage across the shunt. This voltage is provided to high-input impedance measurement circuitry located within the measuring instrument, which outputs a measurement value proportional to the monitored shunt voltage.
This approach has several disadvantages. First, whenever a measurement lead is present, the supply voltage utilized for measurement lead detection is coupled to ground through the two high-value pull-up resistors in series with the low-value resistor shunt and overcurrent fuse, resulting in electrical current flow. This electrical current induces a voltage across the shunt that increases the voltage induced by the current injected into the electrical measuring instrument by the item being tested via the measurement lead, which results in measurement error. While this measurement error can be minimized by increasing the impedances of the high-value pull-up resistors, the error cannot be eliminated.
Second, even when a measurement lead is not present, leakage current flowing between the split jack electrical contacts may be sufficient to cause the lead detect circuit to indicate that a lead is present. In this regard, dirt and humidity can create a leakage path between the electrical contacts of the split jack. When this occurs, the supply voltage is coupled to ground through the series combination of the voltage divider, leakage path, overcurrent fuse, and shunt. In many circuits the effect of such a leakage path is negligible because the impedance of the leakage path is several orders of magnitude larger than any other impedance in the circuit, resulting in nearly all the voltage drop appearing across the leakage path. Unfortunately, this is not true when the impedances of the two high-value pull-up resistors are increased so as to minimize the error current when a measurement lead is present. In this case, the impedance of the leakage path between the split jack electrical contacts may have the same order of magnitude as the high-value pull-up resistors. Accordingly, the leakage current results in a much larger portion of the supply voltage appearing across the two high-value pull-up resistors. When this voltage drop is divided at the voltage divider output, the output voltage is reduced to a value below that of the supply voltage. Such a reduction may result in a false measurement lead detection. One way to reduce the possibility of false measurement lead detection due to leakage current is to reduce the values of the pull-up resistors. Unfortunately, this reduction proportionally increases the measurement error current when a measurement lead is present.
As will be readily appreciated from the foregoing discussion, there is a need for a new and improved measurement lead detect circuit that reduces or eliminates error due to the current induced by a supply voltage utilized during measurement lead detection and reduces the susceptibility to false measurement lead detections due to leakage current. The present invention is directed to fulfilling this need.